string longestPalinSubstring(string s)
{
    if (s.empty()) 
        return "";
    if (s.size() == 1) 
        return s;

    int min_start = 0, max_len = 1;

    for (int i = 0; i < s.size();) {
        if (s.size() - i <= max_len / 2) 
            break;

        int j = i, k = i;
        while (k < s.size()-1 && s[k+1] == s[k]) 
            ++k;
        i = k+1;

        while (k < s.size()-1 && j > 0 && s[k + 1] == s[j - 1]) { 
            ++k; 
            --j; 
        }

        int new_len = k - j + 1;
        if (new_len > max_len) { 
            min_start = j; 
            max_len = new_len; 
        }
    }
    return s.substr(min_start, max_len);
}